1. Field of the Invention
The present invention relates to a pulsed coherent Doppler radar allowing both transmission frequency agility and pulse repetition frequency agility, especially in association with phase encoding.
2. Description of the Prior Art
The practice of frequency agility consists in varying the transmission frequency or pulse repetition frequency of the radar in time, making the radar more difficult to detect in order to improve its resistance to enemy action in, for example, the battle-field.
In conventional radars with frequency agility, high-power oscillator tubes are used in the coherent channels, i.e. the oscillator tube output frequency is memorized by a coherent oscillator known as a "COHO", in order to provide the means of comparison with the signal returned from a target and containing the radio velocity information in the form of its Doppler frequency.
Such radars, however, have many disadvantages. Firstly, the farther the target, the longer the memorization period and the worse the quality of the memorized signal, consequently limiting the extent of ranging although the transmitted power would allow it to be increased.
In addition, the frequency setting time is excessive, since the cavity tuning is mechanical, which often limits the possibility of a frequency jump for each antenna revolution. Moreover, after changing the cavity frequency, the transmission produces a certain degree of instability from one pulse to the next.
In more recent radars, use is made of amplifier systems operating as a coherent channel, i.e. using two ultra-stable oscillators for both transmission and reception.
The validity of this system resides in the low differential phase drift of the oscillators between the instant of transmission and the instant of reception, but in the case of extremely distant targets, this drift is no longer negligible and degrades system quality.
In addition, in order to use frequency agility, it is necessary to multiply the number of oscillators by the number of frequencies to be used, which limits the number of possible frequencies because of increasing cost of the equipment.